Long story short, changing PHP version for Virtual Server Configuration > PHP Versions from PHP 5.6 (installed per https://www.virtualmin.com/documentation/web/multiplephp) back to PHP 5.4.16 didn't want to take any effect, so I proceeded to removing PHP 5.6 from the system running:
yum remove rh-php56*
yum remove rhscl-rh-php56-epel-6-x86_64.noarch
restarted Apache, but the website is showing Internal Server Error. The Apache error log shows relevant lines:
[Wed May 11 16:44:58.366473 2016] [mpm_prefork:notice] [pid 6362] AH00163: Apache/2.4.6 (CentOS) OpenSSL/1.0.1e-fips mod_fcgid/2.3.9 PHP/5.4.16 SVN/1.7.14 mod_perl/2.0.9dev Perl/v5.16.3 configured -- resuming normal operations
[Wed May 11 16:44:58.366505 2016] [core:notice] [pid 6362] AH00094: Command line: '/usr/sbin/httpd -D FOREGROUND'
php5.6.fcgi: line 10: /opt/rh/rh-php56/root/usr/bin/php-cgi: No such file or directory
So it seems to still be looking for PHP 5.6. Rebooting the server didn't help.